1. Field of the Invention
This invention relates to an air-fuel control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which carries out feedback control of the air-fuel ratio of a mixture supplied to the engine, by applying an adaptive control theory thereto.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 7-247886, which feedback-controls the air-fuel ratio of a mixture supplied to the engine by the use of an adaptive controller based on an adaptive control theory, which includes a parameter-adjusting mechanism of a recurrence formula type. In the proposed air-fuel ratio control system, an air-fuel ratio sensor arranged in the exhaust system of the engine detects the air-fuel ratio and supplies the adaptive controller with the detected air-fuel ratio in response to which the adaptive controller carries out the air-fuel ratio feedback control.
When the adaptive control is carried out in synchronism with the combustion cycle of the engine, if there are variations in the air-fuel ratio between the cylinders, the accuracy of the adaptive control can be degraded due to a strong influence of the air-fuel ratio of a particular cylinder. To eliminate this inconvenience, there has been proposed by the present assignee in Japanese Patent Application No. 7-354051, a technique of smoothing the detected air-fuel ratio to be input to the adaptive controller when the engine is in a predetermined operating condition.
However, according to the proposed technique, when the desired air-fuel ratio is changed, the smoothed detected air-fuel ratio input to the adaptive controller cannot accurately agree with the changed desired air-fuel ratio, so that it is impossible to carry out proper adjustment of adaptive parameters for use in the adaptive control, which causes degraded controllability of the air-fuel ratio.